1. Field of the Invention
The invention relates generally to a method and apparatus to correct compass magnetic heading and/or orientation sensing indications to account for gain, offset, and orthogonality errors, as well as reference alignment rotation errors.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98.
Directional field measurement sensors such as inclinometers and magnetometers can have gain, offset, and orthogonality errors, as well as reference alignment rotation errors. If a series of 3 axis sensor readings are taken with a variety of different orientations, the resulting dataset looks like a perfect sphere in the absence of any errors; with all the above errors it looks like an offset, rotated, ellipsoidal surface. Correcting this ellipsoidal surface back to a correctly aligned sphere centered at the origin removes, or at least minimizes, gain, offset, and orthogonality errors, as well as reference alignment rotation errors. As used herein an inclinometer is of a group comprising clinometer, tilt meter, tilt indicator, gradiometer, pitch and roll indicator, accelerometer, liquid capacitive, electrolytic and MEMS based devices of the preceding; for purposes of the instant invention a gyroscope, of one, two or three axes, is included in this group; these terms may be used interchangeably and all cases are meant to be inclusive of the group. The term “sensor”, as used herein, comprises inclinometers, including gyroscopes, and magnetometers and others as may be known by one knowledgeable in the art. In some embodiments a sensor is a rate sensor; a rate sensor may be a gyroscope sensing rate of angular change; a rate sensor may be an angular rate sensor of one, two or three axes of a group comprising but not limited to coriolis vibratory gyro, MEMS angular rate sensor, fiber optic gyroscope, gyrocompass, ring laser gyroscope; a rate sensor may be a magnetometer comprising timing circuits and other components sufficient to measure angular rate of change through a relatively static magnetic environment; a rate sensor may be an accelerometer comprising timing circuits and other components sufficient to measure angular rate of change through a relatively static inertial environment; a rate sensor may be a sensor as may be known by one knowledgeable in the art comprising timing circuits and other components sufficient to measure angular rate of change through a relatively static environment;
One common method of removing all these errors involves employing a calibration test system that orients the sensor at a multitude of known orientations. The resulting set of points can be used to determine the gain, offset, and cross axis terms to reduce the errors. This method may work, but suffers from potential limitations. One limitation is that determining an exact orientation at each measured point can be difficult because of mechanical limitations. Another limitation is that there may be a need to perform a calibration when such equipment is too expensive or not available. For instance, a field calibration to correct for drift or alignment changes would be quite useful, even if the sensors were originally calibrated in a factory test system.
Prior art is found in: Automatic sensor alignment, U.S. Pat. No. 5,245,909, Spacecraft accelerometer auto-alignment, U.S. Pat. No. 4,749,157; Calibration of 3D field sensors; U.S. Pat. No. 7,275,008. All patents, patent applications, and other documents referenced herein are incorporated by reference in their entirety for all purposes, unless otherwise indicated.